JWST Micrometeoroid Avoidance Zone

A micrometeoroid avoidance zone (MAZ), that overlaps part of the JWST field of regard, has been defined. Minimizing JWST pointings in this region will lower the risk of damage from incoming  debris.

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See also: Micrometeoroid Avoidance Zone: Policies and ProceduresAPT Micrometeoroid Avoidance

The potential for micrometeoroid impacts on JWST was predicted pre-launch. Predictions of frequency, particle size, and velocity distributions were thought to not cause significant short-term risk. However, during commissioning, one of the JWST primary mirror segments was damaged by a high velocity piece of space debris. The effect of the damage was noticeable (see Figure 1) , but its impact on the telescope's optical performance was relatively minor. Telescope performance continues to exceed pre-mission predictions with a good bit of margin. (See Science Performance report, sec. 4.7.)

Nonetheless, given the uncertainties and the damage incurred from such an event very early in the mission lifetime, NASA has decided to minimize the need to make observations in directions that enhance the risk. NASA has thus defined a Micrometeoroid Avoidance Zone (MAZ).  By minimizing the time pointed into the MAZ, NASA intends to minimize further degradation of the observatory's image quality. Both the timeline schedulers and users/proposers need to be aware of this new observing guideline.

Figure 1. JWST primary mirror wavefront sensing images showing the micrometeoroid impact from the May 24, 2022 event (right)

The image on the right shows wavefront sensing data that reveals damage from the May 24, 2022, micrometeoroid impact. Local deviations in wavefront on segment C3 can be seen, but the overall impact on the achieved wavefront error was minimal. See section 4.7 of the Science Performance report for more information.

The MAZ defined

As the Earth, and JWST, orbit the sun, the motion in the orbital direction is approximately 30 km/s. While JWST is pointed in this general direction, the momentum of any incoming micrometeoroid will be enhanced by the observatory’s motion, hence increasing the risk of potential for damage (in particular to the primary mirror segments or their coatings). 

The MAZ is defined as a cone of a specified half angle around the orbital motion direction, also referred to as "the ram vector." Starting in Cycle 2, the half angle has been set to 75°. The projection of the MAZ footprint onto the celestial sphere overlaps with JWST’s field of regard (FOR; see Figure 2), which is the portion of the sky available to perform observations at any given time. 

It is NASA’s intention to discourage observations in the MAZ-FOR overlap region to the greatest extent possible. Proposers are encouraged to avoid such observations unless the science use case requires it. Requests for observations in the ram direction will need to be strongly justified, and may be rejected if the demand for such observations is too high.

Figure 2. Field of regard impacts from a micrometeoroid avoidance zone

The graphic on the left shows the nominal field of regard (plain gray area) on the celestial sphere, assuming JWST is sitting at the center of the sphere, with the orbital motion (ram) vector indicated. (Imagine the ram vector not in the plane of the figure but rather pointing out of the page.) The blue oval with lines indicates the anti-sun avoidance region and the blue lined region to the right is the solar avoidance region. 

On the right, the red circular region shows an assumed MAZ, which overlaps with the field of regard. This FOR-MAZ overlap region is a portion of nominal visibility for the observatory that will be avoided as much as possible in planning and scheduling in order to minimize risk from potential micrometeoroid impacts enhanced by the orbital motion of the observatory. 

The Impact of the MAZ on target schedulability

The JWST FOR forms a large torus projected onto the celestial sphere. As JWST orbits the sun, this FOR sweeps across the celestial sphere from west to east. Alternatively, from the perspective of a fixed target on the celestial sphere, a target enters the FOR on the east side and traverses through to the west, as illustrated in Figure 3. 

Figure 3. A graphic showing how targets at ecliptic latitudes above |45°| and below |45°| experience differing impacts from imposing a MAZ

Targets at low ecliptic latitudes (see Target 1) have their visibility interrupted by the anti-sun avoidance region, and thus have 2 periods of visibility during a given cycle. One of these 2 periods (that is, 50% of the visibility) will overlap the MAZ, and scheduling in that region needs to be minimized. Targets at higher ecliptic latitudes (see Target 2) have one longer period of visibility, and <50% of their visibilities are within the MAZ. Target visibilities near the ecliptic poles are unaffected by the MAZ.
Because of the avoidance region toward the anti-sun direction, the details of a target’s visibility depend on the ecliptic latitude of the target, with targets below |45°| ecliptic latitude having 2 visibility windows per year and targets at higher ecliptic latitudes having one longer visibility period. Of course, targets very close to the celestial poles are available throughout the year in a continuous viewing zone (CVZ).

The effect of the MAZ as it is currently defined is to completely cover the leading visibility window for targets below |45°| ecliptic latitude.  Hence, in order to avoid scheduling in the MAZ region, half of these targets’ visibility is nominally unavailable to schedulers. For targets at higher ecliptic latitude, <50% of nominal visibility is lost, with targets near the CVZ not being impacted at all by the MAZ.

For the vast majority of observations of fixed targets without additional constraints, the solution to avoiding observations in the MAZ is to assign long range plan (LRP) windows in the non-MAZ portion of the target’s visibility. To the extent possible, STScI planners will strive to do this, thus minimizing observing when targets are within their MAZ-FOR visibility region.

The observer’s responsibility

Proposers and observers need to be aware that the use of various APT special requirements can actually force scheduling observations into the MAZ portion of their visibility. Two simple examples demonstrate the issues:

  1. A time critical observation using the "BETWEEN DATES" special requirement, where the selected dates force pointing into the MAZ portion of the target's visibility.

  2. Since aperture position angles are tied to specific times of visibility, specifying a position angle (or small range of position angles) onto a particular target’s observation could force the observation to be scheduled in the MAZ.

There are any number of additional timing or position angle constraint applications that could have the same effect on scheduling. Starting with Cycle 2 proposing (APT 2022.7, release date ), APT will flag such possibilities with a warning message. Users receiving this message should inspect the APT MAZ constraint graphic  and consider whether a simple change in their proposal or special requirement choices could mitigate the need for MAZ observing while still accommodating their science goals. 

It is important to remember that MAZ observing is not disallowed but should be minimized. Proposers will be given an opportunity within APT to provide an explanation if observing in the MAZ region is required to reach their science goals; this explanation is for STScI use only and is not included in the proposal PDF seen by peer reviewers. However, users should make every effort to help schedulers avoid the need for MAZ pointing observations when crafting their proposals.

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